Gowtham G - Rectangular tunnels

Rectangular tunnels with rounded corners in soils using the pseudo-static approach

Tunnels in granular soils

The stability of square and rectangular tunnels with rounded corners placed in sandy soil subjected to seismic forces was examined. The pseudo-static method was used in the framework of lower bound finite element limit analysis based on the distribution of stresses in a statically admissible stress field for the stresses generated in the soil due to seismicity. The support pressure, the minimum pressure required for tunnel stability, was calculated as the maximum of the normal stresses exerted by the soil surrounding the tunnel perimeter. The variation of support pressure with tunnel cover depth and the aspect ratio of the tunnel was extensively reported. The support pressure was found to increase for an increase in the seismic coefficients and a decrease in the friction angle of the soil. Also, the support pressure for narrow rectangular tunnels was found to be less than the square or wide tunnels. The stress is noted to be higher along the wall as compared to the roof and base in the case of tall rectangular tunnels and square tunnels, implying the collapse of these tunnels commences from the tunnel walls. In wide rectangular tunnels, the maximum stress occurs along the roof compared to the side wall and base, where the failure begins from the roof.

Gowtham, G., and Sahoo, J.P. Stability of square and rectangular tunnels in sand under seismic loading. Natural Hazards 119, 1863–1881 (2023). https://doi.org/10.1007/s11069-023-06188-3

Tunnels in cohesive-frictional soils

Seismic activity on a tunnel damages the tunnel support systems. The extent of the tunnel damage depends on the soil type, the magnitude of the earthquake acceleration, and the tunnel cover depth. Hence, analyzing the stress induced by the seismic event from the surrounding ground on the tunnel facilitates a safe tunnel design. Based on the pseudostatic method, this study examined the seismic stability of square and rectangular tunnels placed in cohesive-frictional soil. The tunnel collapse load was found using the lower-bound theorem of limit analysis in combination with the finite-element method. From the distribution of stresses along the periphery, the normal stress at each tunnel node was calculated, and the maximum of stresses was reported as the support pressure. Thus, the systems safeguarding the tunnel against devastating lateral earthquake forces are expected to offer the ultimate resistance equal to the maximum normal stress on the tunnel periphery. With the increase in tunnel cover depth, aspect ratio, seismic acceleration coefficients, and a decrease in soil cohesion and friction angle, the support pressure was noted to enhance. The distribution of normal stresses around the tunnel periphery depends on the tunnel geometry, the soil’s shear strength parameters, and the magnitude of earthquake acceleration. For a square tunnel, the magnitude of stress was maximum on the walls, followed by the roof and base, implying that collapse will be more prone from the side walls. However, the rectangular tunnels are noted to be susceptible to collapse from the roof, followed by walls and base.

Gowtham, G., and Sahoo, J.P. (2024). Seismic stability analysis of square and rectangular tunnels in cohesive-frictional soils. Natural Hazards Review, ASCE, 25(3). https://doi.org/10.1061/NHREFO.NHENG-1955

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